Rejuvenation by cell reprogramming: a new horizon in gerontology

Abstract

The discovery of animal cloning and subsequent development of cell reprogramming technology were quantum leaps as they led to the achievement of rejuvenation by cell reprogramming and the emerging view that aging is a reversible epigenetic process. Here, we will first summarize the experimental achievements over the last 7 years in cell and animal rejuvenation. Then, a comparison will be made between the principles of the cumulative DNA damage theory of aging and the basic facts underlying the epigenetic model of aging, including Horvath's epigenetic clock. The third part will apply both models to two natural processes, namely, the setting of the aging clock in the mammalian zygote and the changes in the aging clock along successive generations in mammals. The first study demonstrating that skin fibroblasts from healthy centenarians can be rejuvenated by cell reprogramming was published in 2011 and will be discussed in some detail. Other cell rejuvenation studies in old humans and rodents published afterwards will be very briefly mentioned. The only in vivo study reporting that a number of organs of old progeric mice can be rejuvenated by cyclic partial reprogramming will also be described in some detail. The cumulative DNA damage theory of aging postulates that as an animal ages, toxic reactive oxygen species generated as byproducts of the mitochondria during respiration induce a random and progressive damage in genes thus leading cells to a progressive functional decline. The epigenetic model of aging postulates that there are epigenetic marks of aging that increase with age, leading to a progressive derepression of DNA which in turn causes deregulated expression of genes that disrupt cell function. The cumulative DNA damage model of aging fails to explain the resetting of the aging clock at the time of conception as well as the continued vitality of species as millenia go by. In contrast, the epigenetic model of aging straightforwardly explains both biologic phenomena. A plausible initial application of rejuvenation in vivo would be preventing adult individuals from aging thus eliminating a major risk factor for end of life pathologies. Further, it may allow the gradual achievement of whole body rejuvenation.

Keywords: Aging; Cell reprogramming; Epigenetics; Rejuvenation; Therapeutic potential.

Fig. 1

Rejuvenation by cell reprogramming of fibroblasts from healthy centenarian individuals. In culture, fibroblasts from old individuals display a typical transcriptional signature, different from that of young counterparts as well as shortened telomeres, reduced population-doubling (PD) potential, dysfunctional mitochondria, and higher levels of oxidative stress. When cells were reprogrammed to iPSC with a six-factor cocktail, the above alterations were reversed to embryonic levels. Then, iPSCs were differentiated back to fibroblasts by culture in the presence of an appropriate set of differentiation factors. In the resulting cells, all of the above variables had levels typical of fibroblasts taken from young individuals, see [4] for further details

Fig. 2

Rejuvenation of transgenic progeric mice by cyclic partial cell reprogramming. A polycistronic cassette (4F system) harboring the four Yamanaka genes under the control of a tetracycline-regulatable (Tet On) promoter (a) was transferred to one-cell embryos of C57bl/6 wild-type mice in order to generate transgenic mice harboring the 4F system (4F mice) that were subsequently backcrossed with transgenic progeric mice (LAKI mice). This way, progeric LAKI-4F mice were generated. The antibiotic doxycycline (DOX) binds to the regulatory rtTA protein which then gains affinity for the Tet On promoter and binds to it turning on the Yamanaka genes (b). When DOX is removed from the medium, the rtTA protein dissociates from the promoter and the transgenes become silent again (c). When DOX was added to the drinking water of 2-month-old progeric mice, it turned on the Yamanaka genes and partial cell reprogramming began (d). Two days later, DOX was removed and the Yamanaka genes silenced (e). After a 5-day resting period, DOX was added again for 2 days (f), then removed for 5 days and so on. This cyclic partial reprogramming process rejuvenated some tissues and organs of the mice which survived 50% longer than the original progeric mice, see [16] for further details

Fig. 3

Diagrammatic representation of the cumulative DNA damage theory and the epigenetic model of aging. a Progressive age-related DNA damage that takes place in the genome of cells with age due to environmental insults. ROS, reactive oxygen species. b The upper diagram represents some of the progressive changes in histones H3 and H4 methylation and acetylation during normal aging. Changes in DNA methylation are represented by stemmed asterisks on DNA. The lower diagram represents the chronologic changes that may occur on the same epigenetic marks during OSKM gene-induced rejuvenation/dedifferentiation. Red symbols represent chromatin activating marks whereas black symbols correspond to chromatin-repressor marks. Blue wavy lines represent the gene transcripts

Fig. 4

The two theories at work. a According to the cumulative DNA damage theory, when a hypothetical 25-year-old human couple conceives a new individual, the zygote they conceived inherits the DNA damage of the parental germ cells. (Left enlarged panel) According to the theory, after each generation, DNA damage in the successive zygotes would be accumulated through inherited damage, causing species viability to decline progressively over the centuries, eventually driving them to extinction. (Main diagram) According to the epigenetic model, at the time of fertilization, all the epigenetic marks of parental aging are erased from the zygote’s genome by the reprogramming factors present in the cytoplasm, thus resetting its aging clock back to zero (b, left enlarged panel). Consequently, in each generation, the epigenetic clock of zygotes will restart from zero, thus allowing that complex animal species flourish and diversify over time (b, main diagram)